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Monday, August 1, 2011

All anthropological subfields make important contributions to our defining goal of answering questions about ourselves, but biological anthropology is unique in its ability to reveal our ancient histories. We can all agree that a full and nuanced understanding of any anthropological issue requires an understanding of its historical context. In the case of humans as biological organisms, a biological historical perspective is needed to achieve this because “nothing in biology makes sense except in the light of evolution” (Dobzhansky, 1964 p. 449).

For example, a topic of interest to all anthropologists is the wide diversity of human social and mating systems. Human mating systems run the gamut from polygynous to polyandrous to monogamous. In light of this diversity, it at first seems unlikely that there could be any unifying characteristics of human mating systems. However, one characteristic that might link all human societies is cooperative breeding. The human life history schedule—an average inter-birth interval of 3.5 years combined with a much longer period of nutritional dependency—results in mothers having more than one dependent offspring at a time. As a result, many have suggested that the work of successfully raising multiple, dependent offspring is more than a mother can handle alone, and that cooperative rearing is a necessity for successful human reproduction. Biological anthropology offers both a historical perspective and a comparative approach that can assess whether humans, as a species, can be characterized as cooperative breeders.

Humans use myriad techniques to garner support for themselves and their dependent offspring; can we really be uniformly defined as cooperative breeders? Perhaps. Cooperative breeding in other animals is also defined variably and is achieved by a variety of different mechanisms from hormonal (e.g., Saltzman et al., 2000) to behavioral (e.g., Sparkman et al., 2011). Despite this variation in expression and proximate causation, cooperative breeders are generally characterized by delayed reproduction compared to their close relatives (e.g., Clutton-Brock et al., 1998, meerkats; Armitage, 1999, marmots; Saltzman et al., 2000, marmosets; Sparkman et al., 2011, wolves).

If delayed reproduction is an indicator of a cooperative breeding system, do humans bear this hallmark? Compared to other apes, humans are characterized by early weaning, a long childhood, late age at first reproduction, and short inter-birth intervals. In modern human subsistence societies, weaning occurs around age 3, childhood persists until around age 12, the age at first reproduction is around 19, and the interval between births is about 3.5 years. When we compare this pattern of developmental timing to that of our closest relatives, the chimpanzees, we see that humans do indeed exhibit delayed reproductive maturation. Despite the fact that chimpanzees don’t wean until around age 5 and have a slower inter-birth interval than humans of about 5.5 years, they reach reproductive age much earlier than humans, at about 14 (chimpanzee and human hunter-gatherer data summarized in Kaplan et al., 2000). In sum, even though modern humans wean early and can reproduce quickly once we begin our reproductive careers, we have a prolonged childhood and adolescence, and do not begin reproducing until relatively late in life. These data, coupled with the ethnographic data on non-maternal care of juveniles in human populations (summarized in Kramer, 2010) strongly support the idea that humans are cooperative breeders.

This prompts an additional question: how long have humans been cooperative breeders? Is this characteristic of the human mating system a recent, very modern development? Or was it something we shared with all of our fossil relatives? The answers to these questions can be gleaned from fossil teeth. Dental enamel is the hardest material in the body and mostly comprises inorganic mineral. As a result, teeth preserve extraordinarily well and are abundant in the fossil record. In addition, enamel is laid down on a circadian cycle, which can be quantified in histological specimens, and the day of birth is marked by a visible line in the enamel. As a result, an examination of an individual’s teeth can conclusively establish his or her chronological age at death (among other things). Furthermore, molar eruption roughly coincides with major somatic developmental landmarks such as the cessation of brain growth (first molar, or M1, eruption), the onset of adolescence (M2 eruption), and adulthood/reproductive maturity (M3 eruption).Therefore, given a sufficient sample of individuals at different ages of death, these dental characteristics can provide information on the rate and timing of tooth eruption and coincident growth rates and life history schedules of fossil species.

While inferring life history schedules in fossil taxa is still a challenge (due to the sampling issues inherent in small sample sizes), based on the currently available data, we can draw some general conclusions about the developmental timing of our hominin ancestors and what this might mean about their life histories. A comparison of growth rates derived from dental data of modern humans, extant non-human apes, and fossil hominins reveals that the modern human pattern of growth and development did not characterize early genus Homo. Homo erectus, a species that skeletally resembles modern humans more than any earlier hominins (i.e., it had modern body proportions, a larger brain, and smaller teeth than previous taxa), had a first molar eruption a little bit later than a chimpanzee (chimpanzee 3-4 yr, H. erectus ~4.5 yr), but still earlier than modern H. sapiens (~6 yr). Similarly, the M2 erupted earlier than modern humans (one Homo erectus specimen yields an M2 eruption age of ~8 years, modern H. sapiens ~12 yr). In other words, H. erectus seems to have been more chimpanzee-like in its rate and timing of growth, reaching adulthood relatively earlier than H. sapiens. There are conflicting reports on the dental development of Neandertals, but the most recent research suggests that H. neanderthalensis, which overlapped temporally with H. sapiens and lived until relatively recently (about 30,000 years ago), also matured relatively slower than earlier hominin species, and may have reached adulthood faster than modern humans. Although it is based on a just a few specimens, it appears that Neandertal M1s erupt on the lower end of the modern human spectrum (≤ 6 yr), and the M2s seem to erupt around 7-10 years (Smith et al., 2010). In contrast, fossil specimens of H. sapiens seem to show the same pattern of dental development as contemporary H. sapiens. Altogether, these data suggest that the pattern of delayed reproduction of modern humans originated with us, and not earlier in our evolutionary history.

So, are humans cooperative breeders? The demographic data suggest that we are. The paleoanthropological data further suggest that our modern human pattern of life history arose with our species and did not characterize our closest fossil relatives. Of course, more fossils are needed to refine the estimates of fossil hominin life history schedules, but currently available evidence suggests that cooperative breeding may be a unifying hallmark of our widely varying modern human mating systems, that this has always been since the advent of our species approximately 200,000 years ago, and that it may have distinguished us from our hominin ancestors.